/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Associates a top level Slate Window with a native window, and "natively" parents that window to the specified Slate window. Although the window still a top level window in Slate, it will be considered a child window to the operating system.
a reference to theSWindowthat was just added.
[TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > AddWindowAsNativeChild ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > InSlateWindow, [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > InParentWindow, const bool bShowImmediately )